How to Specify a Switch for Portable Medical Equipment

When specifying switches for portable medical equipment, the two major design considerations are size and reliability. Whereas miniaturization was once the main

Figure 1. As one of the smallest top actuated tactile switches available on the market, the KMT Series switch helps reduce the overall size of hearing aids.

focus of attention, medical equipment manufacturers are now requiring robust, ultra-miniature components in order to suit a variety of field applications. Depending on the nature of the application, even the smallest components used in medical field equipment are susceptible to contaminants and the effects of harsh environments, thereby affecting component reliability.

There are several portable medical equipment applications where the selection and specifying of operation switches (on/off) and special function switches are critical to the performance of the devices. These applications include hearing aids, portable blood pressure cuffs, glucose meters and transdermal patches.

Miniaturization

Development projects involving behind-the-ear hearing aids place a major emphasis on switch miniaturization in order to reduce the overall size of the hearing aid. Utilizing a nano miniature switch can contribute to improved user comfort as well as minimizing the visibility of the device behind the ear. Some switch series, such as the C&K Components KMT Series, deliver this design flexibility with package sizes as small as 2.6ࡩ.0ࡦ.65 mm (0.45 mm body height, 0.20 mm actuator height and 0.13 mm travel).

Similarly, portable blood pressure cuffs also require ultra miniature switches not only for reduced size and weight, but also for ease of assembly. Reduction in weight is a major design criterion when transporting cuffs for field use, while ease of assembly and operation help increase efficiency.

Ultra miniature switches are also being used in transdermal patches to typically deliver post-surgical drugs by reliably triggering the dispensing of pain killers, antibiotics and other drugs through the skin.

Reliability

Reliability is the other major consideration switch manufacturers must take into account when designing for portable medical equipment, particularly in applications where the switch can be exposed to environmental conditions including moisture, humidity, dust and other contaminants.

Switches used in hearing aids are often exposed to various types of moisture, including perspiration generated behind the ear, making it necessary for some level of component sealing. Synthetic sweat can be extremely abrasive to hearing aids and in turn, the switch. If the switch is not designed and manufactured to withstand these specific environments, moisture may enter the hearing aid and corrode the board, resulting in device failure. To prevent corrosion, devices and components can either be sealed or component plating materials can be altered. When sealing is not an option due to size or other constraints, switch manufacturers often turn to alternate plating schemes to combat corrosion.

Figure 2. KMR Series tactile switches are designed in portable and personal medical devices such as hearing aids, blood pressure cuffs and transdermal patches

Because portable equipment is often used in the field, there is no room for error and little time available to fix malfunctioning equipment. For example, portable blood pressure cuffs must work on the first actuation and continue working for a high number of actuations. This makes reliability a primary design concern when considering the selection of a new switch.

Reliability issues are equally important in glucose meter applications. Detect switches in glucose meters require an extremely high cycle count and a high level of reliability in order to initiate the process of analyzing the blood and accurately reading the blood sugar level of a patient.

Also, it is essential that switches in transdermal patch applications operate properly not only to ensure that medication is being dispensed, but that the proper dosage is dispensed on schedule. To prevent accidental switch actuation and dispensing of the drug at an improper time, many designers are specifying self-regulating tactile switches.

Regulations and Specifications Affecting Reliability

In addition to environmental contaminants, regulations and customer specifications also directly affect a component’s reliability from the initial design process. FDA regulations play an important role in the development of all medical equipment. While only the end products need to meet FDA approvals, the consequences of these requirements result in the components needing to meet certain standards as well.

Many components must be cleaned before being packaged in order to eliminate any surface germs. In some cases, switches used in transdermal patches are exposed to gamma rays when the end device is being cleaned. The gamma rays can potentially degrade some of the components within switches, particularly tact switches. This has driven switch manufacturers to develop specifically designed materials to ensure reliability when exposed to gamma rays. These types of constraints impact the overall design time of new medical device components, which can span two to three years.

Along with FDA regulations, medical equipment manufacturers have their own specifications and reliability requirements in terms of cleanliness, traceability and overall design. There are continuing efforts to increase the quality level of components through traceability, for example, requiring component manufacturers to work within highly identifiable batches and specifically designed material flow processes.

In order to meet the component requirements of portable medical equipment, switch manufacturers are continuously evaluating different materials and processes in their switch designs. While the demand for nano miniature switch products with enhanced reliability and lifespan are increasing, so too are clean room assembly requirements as well as the need for zero defect products. Plating materials and packaging processes will also continue to evolve as the portable medical equipment market expands and switch requirements keep pace.